1. Field of the Invention
The present invention relates generally to assays and more specifically to binding assays, such as antibody/hapten or DNA interactions, using nanoporous membranes for increasing the local concentration of the analyte, to improve the overall sensitivity of the assay. This technique can be used with a wide range of labeling schemes, including radio labeling and labeling with magnetic beads.
2. Description of the Related Art
Binding assays, for example immunoassays, are widely used in the food, medical, and pharmaceutical industries as diagnostic tests for a wide range of target molecules. Many binding assays have been produced and marketed since the principle was first developed.
Immunoassays typically exploit the binding capabilities of antibodies. Antibodies are protein molecules which are frequently considered fighters of infections. They fight infections by binding to the infectious material in a specific manner, forming a complex. This is a signal to the organism to reject that complex. Antibodies may also be produced to specifically bind to a wide range of compounds, as a key fits a lock. However other molecules (e.g., chelators, strands of polynucleic acids, receptors including cellular receptors) that are capable of recognizing and selectively binding other molecules may be employed to detect a wide range of species, such as polynucleic acids (DNA or RNA), polypeptides, glycolipids, hormones, polymers, metal ions, and certain low molecular weight organic species including a number of illegal drugs. To be useful in an assay, this recognition event must generate a signal that is macroscopically observable. The method employed to generate such a signal is one way of distinguishing the various types of immunoassays.
The first immunoassay used radioactive labeling. This radioimmunoassay (RIA) is quite sensitive and widely used, but the hazards, expense, and restrictions associated with handling radioactive material makes alternative immunoassays desirable. Recently, enzyme and fluorescence assays have replaced radioimmunoassays. The present inventors and others have developed techniques using magnetic beads as labels for immunoassays. Other known immunoassay labeling techniques use colloids or fluorescent dyes.
An ongoing goal of immunoassay development is improving the lower limit of detection (LLD). Likewise, it is an ongoing goal of immunoassay development to shorten processing time. This is particularly true in order to counter threats of biological warfare and terrorism, as well as other field applications.
Solid supports are used in many immunoassays, typically as adsorbent layers. Many of these, such as nylon and nitrocellulose membranes have pore sizes greater than 25 nm, to amplify the signal by increasing the surface area of the assay.
Some microbiological assays use membranes to separate and concentrate bacteria. These membranes are typically on the order of 200 nm pore size.
Viruses have been identified with aluminum ultrafiltration membranes with 20 nm pores. Organisms immobilized on these membranes have been identified using both specific and nonspecific dyes.
Chemically selective membranes are used in some chemical sensors to pass the analyte through the membrane. Larger molecules are not allowed to pass through the membrane into the internal sensing solution.
Accordingly, it is an object of this invention to selectively detect a wide range of target species, with a high degree of sensitivity.
It is a further object of this invention to selectively detect a wide range of target species, with a short processing time.
It is a further goal of this invention to improve sensitivity to 100 to 1000 times that of the current laboratory standard enzyme-linked immunosorbant assay (ELISA), with processing times at least 10 times shorter than ELISA.
These and additional objects of the invention are accomplished by the structures and processes hereinafter described.
An aspect of the present invention is a sensor for a selected analyte in a test sample having (a) a semipermeable membrane with pores for retaining the analyte, where the membrane has been chemically modified by attachment of membrane modifiers; (b) immunoassay labels which have label binding ligands where these label binding ligands will have a binding affinity for the membrane modifiers in the presence of the analyte, and a measurably different binding affinity for the membrane modifiers in the absence of the analyte; and (c) a label detecting system, for detecting the presence of the labels on the membrane.
Another aspect of the invention is a method for detecting an analyte in a test sample, having the steps: (a) modifying a side of a semipermeable membrane, the membrane having pores for retaining the analyte, with membrane modifiers; (b) placing the test sample in contact with the membrane on the side of the membrane with the membrane modifiers; (c) drawing the test sample through the membrane, osmotically or with the application of differential pressure across the membrane, so that any analyte present in the test sample is drawn towards the modified membrane surface; (d) disposing immunoassay labels on the side of the membrane with the membrane modifiers, where these labels have label binding ligands where these label binding ligands will have a binding affinity for the membrane modifiers in the presence of the analyte, and a measurably different binding affinity for the membrane modifiers in the absence of the analyte; and (e) detecting the presence of the immunoassay labels on the membrane.